(1) Field of the Invention
The present invention relates to methods and apparatus for testing and measurement and more particularly to methods and apparatus for measuring vapor penetration through a porous media.
(2) Brief Description of the Prior Art
The prior art discloses a variety of ways of using a diffusion cell to monitor vapor penetration.
U.S. Pat. No. 4,548,072 to McAndless, for example, discloses a device for monitoring the penetration of vapor from a challenge liquid occluded by a garment material, through the garment material, wherein pressure is applied against the garment material. The vapor which penetrates through the garment material is picked up by an inert carrier gas and detected to determine the protection afforded by the garment material against penetration by the challenge liquid/vapor.
U.S. Pat. No. 4,958,529 to Vestal discloses a gas diffusion cell having a membrane therein separating the cell into an aerosol flow chamber and a sweep gas flow chamber. The effluent is sprayed as an aerosol into the aerosol flow chamber, and vaporized solvent diffuses through the gas membrane into the sweep gas flow chamber while particles of interest are output to the detector. Sweep gas is passed through the sweep gas flow chamber for removing the solvent vapor, and the flow rate of sweep gas is controlled to achieve little or no net flow of gas across the membrane.
U.S. Pat. No. 5,005,403 to Steudle et al. discloses a process for determining the concentration of a substance dissolved in a solvent by means of an osmometer. The osmometer comprises a two-chamber system with a first chamber having an osmosis cell and a second chamber for the measurement solution. The pressure in the osmosis cell is held constant with an incompressible membrane arrangement.
To summarize the prior art, known methods to determine the diffusion coefficient of a vapor through a porous media are generally either for thin membranes or dry and friable media. With particular regard to the soil science and engineering disciplines, diffusion across thin membranes are known. In the soil science and engineering disciplines, however, diffusion coefficients are generally determined for loosely packed soils that tend to be at near dry conditions.
A need, therefore, exists for a method and apparatus measuring the diffusion coefficient of a vapor through a porous material which compensates for differences in moisture content and confining stress in the porous media. A need, in particular, exists for such a method and apparatus which is adapted for use on soils and other similar porous materials.